Thin film composite (TFC) membranes including ultra-thin active layers and porous supports have been widely used in reverse osmosis (RO) or nanofiltration (NF) processes for desalination. The active layer in a TFC membrane controls separation properties, while the support layer enhances mechanical strength.
TFC membranes commonly experience substantial flux declines when exposed to feed water contaminants such as, for example, particulates or suspensions of organic, inorganic or biological materials. The contaminants can cause membrane fouling, which increases the operating pressure required for a constant rate of water production, decreases the service lifetime of the membrane, and increases operating costs. To reduce fouling, the feed water entering the membrane may be pre-treated, or the membrane may be periodically chemically cleaned to remove contaminant deposits. Although pretreatment processes can remove certain foulants such as large particles or biomolecules, dissolved organic matter can remain on the membrane following the pretreatment step.
The membrane surface may be modified by applying a protective coating layer to mitigate fouling, which can reduce the number of cleaning cycles. Anti-fouling coating layers have been made from, for example, nanoparticles, enzymes, and epoxy compounds. Although the above-described materials and techniques can reduce membrane fouling, generally they are not well-suited for large scale manufacturing because additional casting, drying and/or curing steps may be required to form the thin protective layers on a support membrane. In addition, depending on their thickness, these additional protective coating layers can cause a drop in water flux through the membrane.
Consequently, a continuing need exists for filtration membranes, particularly for water purification, having enhanced fouling resistance, anti-microbial properties, and/or enhanced salt rejection properties.